The present invention relates generally to inspection techniques that use optically acquired image signal information to detect defects present on a surface-patterned object to be inspected. More particularly, the invention is directed to an inspection technique for detecting microscopic defects present on a patterned substrate such as a semiconductor wafer.
In defect inspection of a substrate having patterns formed on a surface (i.e., a patterned substrate), defect detection sensitivity depends greatly upon how accurately a defect-diffracted/scattered detection beam of light, or a defect signal, can be detected in distinction from pattern-diffracted/scattered and underlayer-diffracted/scattered detection beams of light, or background light noise. During the inspection of a semiconductor wafer, in particular, the detection of even more microscopic defects is being demanded in response to the progress of further microstructured patterning, and how accurately a very weak defect signal from a microscopic defect can be extracted distinctively from background light noise is a big technological challenge associated with defect inspection.
A vertical structure of a patterned substrate, which is an object to be inspected, and the kinds of defects to be detected are described below per FIG. 2, taking a semiconductor wafer as an example. FIG. 2 uses reference numbers 20 to 35 and 201 to 251 to denote the vertical structure of the semiconductor wafer, and uses reference numbers 261 to 264 to denote the kinds of defects to be detected.
Reference number 20 denotes an element isolation layer, and reference number 202 denotes a structure in which, after trenching of a silicon (Si) substrate 201, the trenches are filled in with silicon oxide (SiO2), which is an insulator, to provide electrical insulating separation between transistor elements formed on the wafer. Reference number 21 denotes a gate and contact layer, and reference number 211 denotes gate electrode portions formed from polysilicon (poly-Si). The gate electrode portions are greatly influential upon transistor performance, weighing heavily in defect inspection as well. Reference number 212 denotes contact portions.
Each of the contact portions is where a transistor region and an electrical interconnect layer formed above the transistor region are interconnected via a metal, such as tungsten (W), that is buried in a hole etched in the insulating film (silicon dioxide: SiO2). The interconnect layers 22 to 25 form a circuit. These layers are each filled in with an insulating film such as silicon dioxide (SiO2). Reference number 22 denotes a first interconnect layer, which includes a first interconnect portion 221 for planar interconnection. A first via portion 222 is where the transistor region and an electrical interconnect layer formed further above the transistor region are interconnected via a metal buried in a hole etched in an insulating film such as silicon dioxide (SiO2). Reference number 23 denotes a second interconnect layer, which includes a second interconnect portion 231 and a second via portion 232. Similarly, reference number 24 denotes a third interconnect layer, which includes a third interconnect portion 241 and a third via portion 242. Reference number 25 denotes a fourth interconnect layer, which includes a fourth interconnect portion 251.
The interconnect portion of each interconnect layer is formed from a material including a metal such as aluminum (Al) or copper (Cu). The metal buried in the via portion is formed from tungsten (W), copper (Cu), or the like.
The defects to be detected are, for example, a scratch 261, a short circuit 262 and electrical disconnection 264 that are both a pattern defect, and contamination 263.
FIG. 3 is an explanatory diagram of the steps, materials, and typical defects of each layer of the semiconductor device shown in FIG. 2. The layers of the semiconductor device are formed through various steps. These steps include: the step of depositing the material which forms the layer; the step of forming a resist pattern by lithography; the step of removing the layer-deposited material by etching it along the formed resist pattern; and chemical mechanical polishing (CMP) for planarization.
The materials used in each layer and each fabrication step of the semiconductor device are diverse. The kinds of defects to be detected also vary from step to step; in the deposition step, they may be contamination, in the lithographic step for pattern formation and in the etching step, they may be contamination and pattern defects, and in the CMP step for polishing, they may be contamination and scratches.
As described per FIGS. 2 and 3, patterns of various shapes and materials are involved in semiconductor wafer inspection, and defects of various kinds are detected. Inspection devices are configured so that a plurality of detection parameters can be set to obtain optimal defect detection sensitivity according to the particular shape and material of the pattern or the kind of defect to be detected.
As described in JP-A-1997-304289 and JP-A-2007-524832, for example, semiconductor wafer defect inspection devices of a darkfield optical type that are used to inspect defects and contamination present on a substrate with patterns formed on a surface are constructed to illuminate the substrate from an oblique direction and converge the light scattered from the defects, instead of converging via an objective lens the light regularly reflected from the substrate. These inspection devices are also configured so that the light diffracted/scattered from a pattern or underlayer formed on the substrate will be converged via the objective lens, then intensity-reduced by a polarizing filter and/or a spatial filter, and received by a sensor.
With the above configurations, the defect inspection devices of the darkfield optical type can generate an inspection image with a defect represented explicitly as a luminescent spot against a dark background. Therefore, even if image resolution is too high, that is, a sensor pixel size on the sample substrate surface is too large (but up to 0.3 μm), for a minimum size of defects to be detected, the devices can detect smaller defects, for example of 0.1 μm or less in diameter. Since defect inspection devices of the darkfield optical type have such a feature, they are widely used as high-speed high-sensitivity inspection devices on semiconductor device manufacturing lines.
Semiconductor wafer defect inspection devices of the future will be required to have an ability to detect even more microscopic defects with the progress of further device-pattern microstructuring. To respond to this tendency, the optical systems in the patterned-wafer defect inspection devices of the darkfield optical type need to contain appropriate measures against the following several problems.
One of the problems is how to augment a detection aperture (numerical aperture: NA) of the optical system to detect more efficiently the very weak light scattered from microscopic defects. During patterned-wafer defect inspection, however, it is necessary to detect the defect-scattered light in distinction from the light diffracted/scattered from the patterns or underlayer of the wafer. If the detection aperture is merely augmented, although signal intensity of the defect-scattered light will be increased, noise components of the light diffracted/scattered from the patterns or the underlayer will also increase and detection sensitivity of the defect will be difficult to improve.
To cope with these problems, it is effective to utilize a difference in directionality between the defect-scattered light and the pattern- or underlayer-diffracted/scattered light. More specifically, it is effective to detect scattered light in a widest possible range from a plurality of different directions and conduct defect detection using scattered-light images obtained. For example, JP-A-1997-304289 (Patent Document 1) discloses a technique for inspecting defects by detecting scattered light from a plurality of directions. In addition, JP-A-2007-524832 (Patent Document 2) discloses a technique for inspecting defects using the scattered light acquired by a converging optical system placed in an upward direction and oblique direction of a substrate to be inspected. Furthermore, JP-A-2004-177284 (Patent Document 3) discloses a technique for inspecting defects using scattered-light images acquired by an imaging optical system placed in an upward direction and oblique direction of a substrate to be inspected.
Furthermore, JP-A-2008-241688 (Patent Document 4) discloses a technique used to inspect defects by changing an angle of a reflecting mirror positioned between a substrate to be inspected and a detection optical system placed above the substrate, and thereby acquiring images of scattered light from a plurality of directions.
Furthermore, JP-A-2010-54395 (Patent Document 5) discloses a technique used to inspect defects by placing a plurality of reflecting mirrors between a substrate to be inspected and a detection optical system placed above the substrate, and thereby acquiring images of scattered light from a plurality of directions. Moreover, JP-A-2008-261790 (Patent Document 6) discloses a technique for extending a scattered-light detection range by cutting off two end portions of each of circular lenses and using these lenses as part of a detection optical system for detecting scattered light from a plurality of directions. Besides, JP-A-2009-53132 (Patent Document 7) discloses a technique for inspecting defects by conducting comparative processing of scattered-light images acquired from a plurality of directions.
If detectability of a detection optical system is enhanced in an attempt to detect finer defects, such changes as in ambient temperature and in atmospheric pressure will change imaging performance of the detection optical system, resulting in defect detection sensitivity decreasing. Techniques for improving this problem are described in, for example, JP-A-2002-90311, JP-A-2007-248086, and JP-A-2008-249571 (Patent Documents 8, 9, and 10). The techniques disclosed in Patent Documents 8 and 9 relate to correcting changes in imaging position due to changes in temperature and atmospheric pressure. The technique disclosed in Patent Document 10 relates to controlling an internal temperature of an inspection device.
In connection with a scattered-laser-light detection type of defect inspection, “Principles of Optics” (M. Born, E. Wolf), Cambridge University Press, pp. 774-785, (1999) (Non-Patent Document 1) introduces the fact that intensity of a scattered-light signal from a microscopic object whose diameter or radius is smaller than a wavelength of light decreases inversely with the sixth power of a size of the object and increases in proportion to the fourth power of illumination wavelength.
In addition, the relational expression representing the relationship between changes in ambient temperature and ambient air pressure and a change in the reflective index of air is shown in “The Reflective Index of Air” (Bengt Edlen), Metrologia vol. 2, No. 2, pp. 71-80, (1966) (Non-Patent Document 2).
Furthermore, the relational expression representing the relationship between a change in wavelength and a change in the reflective index of a lens material is shown in “Zur Erklarung der abnormen Farbenfolge im Spectrum einiger Substanzen” (Wolfgang Sellmeier), Annalen der Physik and Chemie, pp. 272-282, (1871) (Non-Patent Document 3).